In the tires of heavy goods vehicles, the carcass reinforcement is generally anchored on either side in the area of the bead and is surmounted radially by a crown reinforcement made up of at least two layers that are superimposed and formed of threads or cords which are parallel in each layer and crossed from one layer to the next, forming angles of between 10° and 45° with the circumferential direction. Said working layers that form the working reinforcement may be further covered by at least one layer, referred to as the protective layer, formed by reinforcing elements which are advantageously metallic and extensible and are referred to as elastic reinforcing elements. It may also comprise a layer of metal threads or cords having low extensibility that form an angle of between 45° and 90° with the circumferential direction, this layer, referred to as the triangulation layer, being radially located between the carcass reinforcement and the first crown layer, referred to as the working layer, that is formed by parallel threads or cords lying at angles not exceeding 45° in absolute terms. The triangulation layer forms a triangulated reinforcement with at least said working layer, this reinforcement having low deformation under the various stresses which it undergoes, the triangulation layer essentially serving to absorb the transverse compressive forces that act on all the reinforcing elements in the crown area of the tire.
Cords are said to be inextensible when said cords exhibit, under a tensile force equal to 10% of the breaking force, a relative elongation at most equal to 0.2%.
Cords are said to be elastic when said cords exhibit, under a tensile force equal to the breaking load, a relative elongation at least equal to 3% with a maximum tangent modulus of less than 150 GPa.
Circumferential reinforcing elements are reinforcing elements which form angles with the circumferential direction in the range +2.5°, −2.5° around 0°.
The circumferential direction of the tire, or longitudinal direction, is the direction that corresponds to the periphery of the tire and is defined by the direction in which the tire runs.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular thereto.
The axis of rotation of the tire is the axis about which it turns in normal use.
A radial or meridian plane is a plane which contains the axis of rotation of the tire.
The circumferential mid-plane, or equatorial plane, is a plane which is perpendicular to the axis of rotation of the tire and divides the tire into two halves.
For metal wires or cords, force at break (maximum load in N), breaking strength (in MPa) and elongation at break (total elongation in %) are measured under tension in accordance with standard ISO 6892, 1984.
Certain present-day tires, referred to as “road tires”, are intended to run at high average speeds and over increasingly long journeys, because of improvements to the road network and the growth of motorway networks worldwide. The combined conditions under which such a tire is called upon to run without any doubt make possible an increase in the number of miles travelled, the wear on the tire being reduced. This increase in life in terms of milage, combined with the fact that such conditions of use are likely, under heavy load, to result in relatively high crown temperatures, dictates the need for an at least proportional increase in the durability of the crown reinforcement of the tires.
This is because stresses are present in the crown reinforcement; more particularly, there are shear stresses between the crown layers which, in the case of an excessive rise in the operating temperature at the ends of the axially shortest crown layer, result in the appearance and propagation of cracks in the rubber at said ends. The same problem exists in the case of edges of two layers of reinforcing elements, said other layer not necessarily being radially adjacent to the first layer.
In order to improve the endurance of the crown reinforcement of the tires, the French application FR 2 728 510 proposes arranging, on the one hand, between the carcass reinforcement and the crown reinforcement working ply that is radially closest to the axis of rotation, an axially continuous ply which is formed of inextensible metal cords that form an angle at least equal to 60° with the circumferential direction and of which the axial width is at least equal to the axial width of the shortest working crown ply and, on the other hand, between the two working crown plies, an additional ply formed of metal elements that are oriented substantially parallel to the circumferential direction.
In addition, the French application WO 99/24269 notably proposes, on each side of the equatorial plane and in the immediate axial continuation of the additional ply of reinforcing elements substantially parallel to the circumferential direction, that the two working crown plies formed of reinforcing elements crossed from one ply to the next be coupled over a certain axial distance and then uncoupled using profiled elements of rubber compound over at least the remainder of the width that said two working plies have in common.
The layer of circumferential reinforcing elements is usually formed by at least one metal cord wound to form a turn of which the angle of lay with respect to the circumferential direction is less than 8°. The cords initially manufactured are coated with a rubber compound before being laid. This rubber compound will then penetrate the cord under the effect of the pressure and temperature during the curing of the tire.
The document WO 07/003548 further proposes a crown reinforcement comprising, in each shoulder, at least one layer of reinforcing elements that are mutually parallel in the layer and are oriented circumferentially.
The results obtained with these types of embodiment are frequently satisfactory in terms of endurance and wear in the case of prolonged road running at high speed. However, it would seem that, under certain running conditions, notably in the case of overload, certain tires sometimes exhibit weaknesses in terms of endurance of the crown reinforcement.